This invention relates to electroluminescent devices, especially those that employ an organic material for light emission.
Electroluminescent devices that employ an organic material for light emission are described in PCT/WO90/13148 and U.S. Pat. No. 4,539,507, the contents of both of which are incorporated herein by reference. The basic structure of these devices is a light-emissive organic layer, for instance a film of a poly(p-phenylenevinylene) (xe2x80x9cPPVxe2x80x9d), sandwiched between two electrodes. One of the electrodes (the cathode) injects negative charge carriers (electrons) and the other electrode (the anode) injects positive charge carriers (holes). The electrons and holes recombine in the organic layer generating photons. In PCT/WO90/13148 the organic light emissive material is a polymer. In U.S. Pat. No. 4,539,507 the organic light emissive material is of the class known as small molecule materials, such as tris-(8-hydroxyquinolino)aluminium (xe2x80x9cAlq3xe2x80x9d). In a practical device, one of the electrodes is typically transparent, to allow the photons to escape the device.
As a preliminary point, it should be noted that the values stated here for energy levels, workfunctions etc. are generally illustrative rather than absolute. The workfunction of ITO can vary widely. Numbers quoted in the literature suggest a range between 4 and 5.2 eV. The 4.8 eV value used here serves as an illustrative rather than an absolute value. The applicant has carried out Kelvin probe measurements which suggest that 4.8 eV is a reasonable value. However, it is well known that the actual value can depend on ITO deposition process and history. For organic semiconductors important characteristics are the binding energies, measured with respect to the vacuum level of the electronic energy levels, particularly the xe2x80x9chighest occupied molecular orbitalxe2x80x9d (xe2x80x9cHOMOxe2x80x9d) and xe2x80x9clowest unoccupied molecular orbitalxe2x80x9d (xe2x80x9cLUMOxe2x80x9d) levels. These can be estimated from measurements of photoemission and particularly measurements of the electrochemical potentials for oxidation and reduction. It is well understood in the field that such energies are affected by a number of factors, such as the local environment near an interface, so the use of such values is indicative rather than quantitative.
These devices have great potential for displays. However, there are several significant problems. One is to make the device efficient, particularly as measured by its external power efficiency and its external quantum efficiency. Another is to optimise (e.g. to reduce) the voltage at which peak efficiency is obtained. Another is to stabilise the voltage characteristics of the device over time.
FIG. 1 shows a cross section of a typical device. FIG. 2 shows the energy levels across the device. The anode 1 is a layer of transparent indium-tin oxide (xe2x80x9cITOxe2x80x9d) with a workfunction of 4.8 eV. The cathode 2 is a Ca:Al layer (a calcium layer capped with aluminium) with a workfunction (for the calcium at the interface with the light emissive layer) of 2.9 eV. Between the electrodes is a light emissive layer 3 of poly (2,7-(9,9-di-n-octylfluorene) (xe2x80x9cF8xe2x80x9d) doped with 5% poly-(2.7-(9,9di-n-octylfluorene)-3,6-benzothiadiazole) (xe2x80x9cF8BTxe2x80x9d), having a LUMO energy level 4 at around 2.8 eV and a HOMO energy level 5 at around 5.8 eV. (From now on the term xe2x80x9c5BTF8xe2x80x9d will be used to refer to this doped emissive layer blend). The emitter dopant LUMO and HOMO levels are around 3.4 and 5.8 eV respectively. FIG. 3 adopts a convention that the HOMO and LUMO energy levels for the dopant (F8BT) of the blend are shown by means of a rectangle inserted in the zone that corresponds to the major component (F8) of the blend. In this convention the width and lateral position of the rectangle has no particular meaning, and where a blend comprise three or more materials then two or more inserted rectangles are used. Holes and electrons that are injected into the device recombine radiatively in the 5BTF8 layer. An important feature of the device is the hole transport layer 6 of poly(styrenesulphonic acid) doped poly(ethylenedioxythiophene) (xe2x80x9cPEDOT:PSSxe2x80x9d). This provides an intermediate ionisation potential a little above 4.8 eV, which helps the holes injected from the ITO to reach the HOMO level in the F8. However, there is still a large barrier (approximately 1.0 eV) between the hole transport layer and the light emissive layer. The presence of high barriers is undesirable, for example because it may increase the drive voltage, build up high internal fields or cause accumulation of holes. One view is also that accumulation of charge at an interface is undesirable because it can promote chemical reactions between the polymer and contaminants, leading to conjugation reduction or deep localised states that may then be charged by the accumulation layer. The charge xe2x80x9ctrappingxe2x80x9d is believed to result in higher bias being required to pass the same current through the device, leading to a relatively rapid voltage increase with time, as the device is used.
It is well-known to use oxygen plasma treatment to clean substrates, and especially to remove organic material. It is also well known that such plasma treatment of ITO can be used to modify the ITO""s work function and potentially reduce the hole injection barrier. (See, for example, WO 97/48115).
Processes have been described for the formation of ultra-thin films with to layer-by-layer control. For example, W. B. Stockton and M. F. Rubner, xe2x80x9cMolecular-level processing of conjugated polymers. 4. Layer-by-layer manipulation of polyaniline via hydrogen-bonding interactions,xe2x80x9d Macromolecules, Vol. 30, pp. 2717-2725, 1997 describes polymer self-assembly via hydrogen bonding interactions; Y. Shimazaki, M. Mitsuishi, S. Ito and M. Yamamoto, xe2x80x9cPreparation of the layer-by-layer deposited ultrathin film based on the charge-transfer interaction,xe2x80x9d Langmuir, Vol. 13, pp. 1385-1387, 1997 describes polymer self-assembly via charge-transfer interactions; A. C. Fou and M. F. Rubner, xe2x80x9cMolecular-level processing of conjugated polymers. 2. Layer-by-layer manipulation of in-situ polymerized p-type doped conducting polymers,xe2x80x9d Macromolecules, Vol. 28, pp. 7115-7120, 1995 describes a process for ultra-thin film formation by active in-situ polymerization; M. Ando, Y. Watanabe, T. Iyoda, K Honda and T Shimidzu, xe2x80x9cSynthesis of conducting polymer Langmuir-Blodgett multilayers,xe2x80x9d Thin Solid Films, Vol. 179, pp. 225-231, 1989 describes a Langmuir-Blodgett deposition process; and K. Kaneto, K. Yoshino and Y. Inuishi, xe2x80x9cElectrical and optical properties of polythiophene prepared by electrochemical polymerization,xe2x80x9d Solid State Communications, Vol. 46, pp. 389-391, 1983 describe electrochemical polymerization on conducting substrates. None of these documents describes gradation of charge transport layers for light emissive devices.
Further details of the manufacture of self-assembled polymer interlayers are given in our co-pending PCT patent application number PCT/GB98/02671, the entire contents of which are incorporated herein by reference. It will be apparent that the techniques described in this application can be combined in various ways with those in that prior application.
According to a first aspect of the present invention there is provided a method of forming an electroluminescent device, comprising: forming a first charge carrier injecting layer for injecting charge carriers of a first polarity; forming an organic charge carrier transport layer over the first charge carrier injecting layer, the transport layer having an electrical and/or optical property which varies across the thickness of the transport layer; forming an organic light emissive layer over the transport layer; and forming a second charge carrier injecting layer over the light emissive layer for injecting charge carriers of a second polarity.
The step of forming a transport layer suitably comprises steps of first depositing the transport layer and then processing the transport layer to create the variation in the electronic and/or optical property(ies) across the thickness of the transport layer. An alternative is to deposit the transport layer in such a way that it has the required properties when deposited, for example by using self-assembly or other deposition techniques. Examples of other layer-by-layer polymer deposition techniques include polymer self-assembly by electrostatic, hydrogen-bonding or donor-acceptor interactions, Langmuir-Blodgett assembly methods, and in-situ polymerisation electrochemical preparation techniques. If small molecule materials were used instead of polymers then small molecule self-assembly reactions that lead to the formation of stratified structures could be used.
Examples of the said electronic and optical property(ies) include one or more energy levels and/or energy level distributions, which are suitably responsible for transport of charge carriers such as holes or electrons. In the case of holes these may (for instance) be a HOMO level, a valence band, an ionisation potential an acceptor doping level or trap or other states close to (e.g.) an ionisation potential. In the case of electrons these may (for instance) be a LUMO level, a conduction band, an electron affinity, donor states or trap or other states close to (e.g.) an electron affinity. Other such properties are the bandgap or optical gap. One or more of these properties may vary across the thickness of the transport layer. The energy level may be for accepting charge carriers of the first polarity or for accepting charge carriers of the second polarity.
The charge carriers of a first polarity are suitably positive charge carriers, in which case the transport layer is a positive charge carrier transport layer.
According to a second aspect of the present invention there is provided a method of forming an electroluminescent device, comprising: forming a first charge carrier injecting layer for injecting positive charge carriers; forming an organic light emissive layer over the transport layer, an electrical and/or electronic property of the light emissive layer varying across the thickness of the emissive layer; and forming a second charge carrier injecting layer over the light emissive layer for injecting negative charge carriers. The said property is suitably of the types set out in detail above.
The step of forming a light emissive layer suitably comprises steps of first depositing the emissive layer and then processing the emissive layer to create the variation in the said property (e.g. its ionisation potential or band gap) across its thickness.
The step of processing or modifying the emissive layer or, in the first aspect of the invention, the transport layer, preferably comprises exposing the emissive or transport layer to an agent that causes modification of the electronic characteristics of the layer. One possibility is that the agent could be a reactive agent, which suitably promotes a chemical reaction in the transport layer. Preferably the conditions of the reaction are such that the degree of reaction varies through the emissive or transport layer so as to provide the variation in the ionisation potential. Preferably one major surface of the emissive or transport layer is exposed to the agent.
The reaction may suitably be an oxidation reaction or a reduction reaction (which could cause de-doping), especially in the case of the transport layer. The agent may be an oxidising agent, for example oxygen. The agent is suitably in the form of a plasma. One preferred reactive oxidising agent is an oxygen plasma. The degree of oxidation, reduction or dedoping preferably varies through the thickness of the transport layer or the emissive layer, suitably leading to the variation in the electronic/optical properties. The plasma preferably also comprises an inert gas, suitably for cooling purposes
The emissive or transport layer suitably comprises a conjugated material. Then, the step of creating the variation in the electronic/optical property(ies) preferably comprises reducing the degree of conjugation of the conjugated material.
The electronic/optical property(ies) preferably varies continuously or substantially continuously through the emissive or transport layer. In the first aspect of the invention, in a direction from the first charge carrier injecting layer to the light emissive layer the ionisation potential preferably varies away from the conduction band (Fermi level) of the first charge carrier injecting layer towards the appropriate HOMO or LUMO level of the light emissive layer. In the second aspect of the invention the optical gap of the light emissive layer preferably changes (most preferably widens) in a direction from the first charge carrier injecting layer to the second charge carrier injecting layer.
According to a third aspect of the present invention there is provided an electroluminescent device comprising: a first charge carrier injecting layer for injecting positive charge carriers; a second charge carrier injecting layer for injecting negative charge carriers; an organic light emissive layer located between the charge carrier injecting layers; and an organic charge carrier transport layer located between the light emissive layer and one of the charge carrier injecting layers, and comprising an organic material having an energy level for accepting positive charge carriers from the said one of the charge carrier injecting layers which varies across the thickness of the transport layer.
The said energy level for accepting positive charge carriers from the said one of the charge carrier injecting layers may be replaced or supplemented by an organic charge carrier transport layer located between the light emissive layer and the other of the charge carrier injecting layers, and comprising an organic material having an energy level for accepting negative charge carriers from the said other of the charge carrier injecting layers which varies across the thickness of the transport layer.
According to a fourth aspect of the present invention there is, provided an electroluminescent device comprising: a first charge carrier injecting layer for injecting positive charge carriers; a second charge carrier injecting layer for injecting negative charge carriers; and an organic light emissive layer located between the charge carrier injecting layers, the optical gap of the light emissive layer varying across the thickness of the emissive layer
According to a fifth aspect of the present invention there is provided a method of forming an electroluminescent device comprising a first charge carrier injecting layer for injecting charge carriers of a first polarity, a second charge carrier injecting layer for injecting charge carriers of a second polarity and at least one organic layer located between the charge carrier injecting layers, the method comprising at least partially oxidising or reducing the organic layer. Oxidation may, for example, be by exposure to a reactive oxidising agent such as an oxygen plasma or by photooxidation. Reduction may, for example, be by exposure to a reactive reducing agent. Suitably a major surface of the transport layer or the light emissive layer is exposed to the reactive oxidising or reducing agent or the light used for photo-oxidation.
According to a sixth aspect of the present invention there is provided a method of forming a layer comprising an organic compound, the layer having at least one property that varies across its thickness, the method comprising forming a series of sub-layers, differing in that property. The property is preferably a material property such as ionisation potential, electron affinity or bandgap or in general an electrical and/or optical property as set out in more detail above. The compound may be a polymer, an otigomer or a small molecular compound. A polymer is preferred. The sub-layers may be deposited with the said differing properties or may be deposited and then modified to gain the said properties. The sub-layers are preferably deposited by a self-assembly-type method, e.g. using Langmuir-Blodgett techniques or self-assembly techniques that make use of electrostatic, hydrogen and/or covalent bonding effects. Another route may be to use blends of materials that on deposition phase-separate into, multilayers, e.g. bilayers. A preferred route is a polyelectrolyte self-assembly method in which the sub-layers are constituted in the form of bilayers. The layer may be a charge transport layer. The layer may form part of an electronic device, such as a light emitting device, preferably a light emitting device that uses an organic material for light emission.
Some preferred materials for all aspects of the present invention are as follows:
One of the charge carrier injecting layers (the hole injecting layer) preferably has a work function of greater than 4.3 eV. That layer may comprise ITO. The other charge carrier injecting layer (the electron injecting layer) preferably has a work function less than 3.5 eV. That layer may suitably comprise calcium, lithium, samarium, ytterbium, terbium, barium or an alloy comprising one or more of those metals with or without another metal such as aluminium. At least one of the electrode layers is suitably light transmissive, and preferably transparent, suitably at the frequency of light emission from the device.
The transport layer may suitably comprise one or more polymers such as poly(styrenesulphonic acid) doped poly(ethylenedioxythiophene) and/or poly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-(4-imino(benzoic acid))-1,4-phenylene-(4-imino(benzoic acid))-1,4-phenylene)) (xe2x80x9cBFAxe2x80x9d) (see FIG. 3) and/or polyaniline (doped, undoped or partially doped) and/or PPV.
The light emissive layer may comprise one or more organic materials, suitably polymers, preferably conjugated or partially conjugated polymers. Suitable materials include PPV, poly(2-methoxy-5-(2xe2x80x2-ethyl)hexyloxyphenylene-vinylene) (xe2x80x9cMEH-PPVxe2x80x9d), a PPV-derivative (e.g. a di-alkoxy or di-alkyl derivative), a polyfluorene and/or a co-polymer incorporating polyfluorene segments, PPVs and/or related co-polymers, poly (2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-((4-secbutylphenyl)imino)-1,4-phenylene)) (xe2x80x9cTFBxe2x80x9d) (see FIG. 3), poly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-((4-methylphenyl)imino)-1,4-phenylene-((4-methylphenyl)imino)-1,4-phenylene)) (xe2x80x9cPFMxe2x80x9d) (see FIG. 3), poly(2,7-(9,9-di-n-octylfluorene)-(1,4phenylene-((4-methoxyphenyl)imino)-1,4-phenylene-((4-methoxyphenyl)imino)-1,4-phenylene)) (xe2x80x9cPFMOxe2x80x9d) (see FIG. 3), F8 or F8BT. Alternative materials include organic molecular light-emitting materials, e.g. Alq3, or any other small sublimed or solution processed molecule or conjugated polymer electroluminescent material as known in the prior art.
Other materials could be used.
The effect of the variation in the electrical or optical properties (e.g. energy level or energy level distribution) of the transport layer and/or the step of modifying the transport layer by (for example) oxidation is suitably to improve transport of the charge carriers of one polarity between the electrode and the emissive layer and/or to impede the transport of charge carriers of the other polarity between the emissive layer and the electrode, and thereby improve the efficiency of the device. It is preferred that at least one energy level of the transport layer is such as to provide a barrier to and/or inhibit passage of charge carriers of one polarity through the device, for example by inhibiting passage of such charge carriers across the interface from the emissive layer; in contrast, passage of charge carriers of the other polarity across that interface is preferably favoured.